In a Wavelength Division Multiplexing (WDM) transmission system, a plurality of optical signals are transmitted in parallel via a single optical fiber utilizing multiple wavelengths which are different from each other. Therefore, WDM is one of the important techniques to implement a high capacity transmission line.
In general, characteristics of an optical fiber and an optical amplifier, which transmits and amplifies optical signals respectively, have a wavelength dependence. Therefore, if a transmission power of each wavelength is equalized when a WDM light is transmitted from a transmitting apparatus of a WDM transmission system, a noise amount may differ with respect to wavelength in a receiving apparatus which receives the WDM light. In other words, in this case, some of the plurality of channels transmitted by the WDM light may deteriorate in quality. Consequently, a pre-emphasis (or also referred to as “equalization”) technique is proposed which adjusts a transmission power of each wavelength in a transmitting apparatus so that a noise of each channel of a WDM light may be equalized in a receiving apparatus.
FIG. 1 is a diagram illustrating a configuration of a WDM transmission system having a pre-emphasis function. In FIG. 1, a transmitter station 1 has optical transmitters (TXP1-TXP40), variable optical attenuators (VOA), and an optical multiplexer. The optical transmitters (TXP1-TXP40) output optical signals having wavelengths which differ from each other. The variable optical attenuators (VOA) adjust powers of optical signals output from corresponding optical transmitters. The optical multiplexer generates WDM light by multiplexing a plurality of optical signals. On the other hand, a receiver station 2 has an optical demultiplexer, optical receivers (RXP1-RXP40), and an optical spectrum analyzer. The optical demultiplexer demultiplexes a plurality of optical signals, which are included in the WDM light, into each wavelength. The optical receivers (RXP1-RXP40) receive corresponding optical signals. The optical spectrum analyzer measures an optical spectrum of the received WDM light.
The transmitter station 1 and the receiver station 2 are connected via an optical fiber. Also, one or more optical repeater stations (REP1-REP3) may be implemented between the transmitter station 1 and the receiver station 2.
In the WDM transmission system configured as described above, the receiver station 2 generates noise information for each channel on the basis of the optical spectrum measured by using the optical spectrum analyzer. The noise information is, for example, an optical S/N ratio (a ratio of the optical signal level to the noise level). The noise information is transmitted to the transmitter station 1. Then, the transmitter station 1 utilize the noise information to adjust the variable optical attenuators (VOA). In this way, a pre-emphasis control may be achieved to equalize a noise of each channel of the WDM light in the receiver station 2. Such a pre-emphasis technique is described, for example, by Japanese Laid-open Patent Publication No. 2002-57624 (Patent document 1).
FIG. 2 is a diagram illustrating an optical spectrum of WDM light. In FIG. 2, bit rates of transmission signals are 10 Gbit/s and 20 Gbit/s, respectively. Further, respective channels are spaced at 100 GHz. The optical S/N ratio is obtained by measuring a peak level (Psig) of an optical signal and a noise level (Pase).
However, as a bit rate of a signal increases, a sideband of an optical spectrum broadens. In general, when a bit rate doubles, its sideband width also doubles. Therefore, if a bit rate becomes high (20 Gbit/s in FIG. 2), a noise level may not be measured accurately. Thus, the optical S/N ratio is not measured accurately.
A method to solve these problems is described by, for example, Japanese Laid-open Patent Publication No. 02-319725 (Patent document 2). Hereinafter, a method described in the patent document 2 will be explained with reference to FIG. 3. Here, channels ch1-ch8 are allocated adjacent to each other.
In order to obtain an optical S/N ratio for each channel, an optical signal level of each channel and a noise level (ASE level) in an adjacent wavelength of each channel are measured. However, if a bit rate of the signal is high and a sideband of an optical spectrum of each channel broadens, the noise level in an adjacent wavelength of each channel is not measured correctly. Therefore, in the method described in the patent document 2, firstly channels ch2, ch4, ch6, and ch8 are suspended and optical S/N ratios for channels ch1, ch3, ch5, and ch7 are measured. Then channels ch1, ch3, ch5, and ch7 are suspended and optical S/N ratios for channels ch2, ch4, ch6, and ch8 are measured.
However, this method requires to suspend temporarily a part of the channels to measure the noise of WDM light, and thus it is difficult to perform a pre-emphasis control during the operation of a WDM transmission system.
In addition, in a WDM transmission system, signals having different bit rates may sometimes be transmitted in parallel. In this case, it may be more difficult to measure an optical S/N ratio of each channel accurately while operating a WDM transmission system.
The above-mentioned problem will not occur if spacings of the channels are broadened. However, if such spacings are broadened, a communication capacity per optical fiber inevitably decreases.